As a cathode active material contained in a positive electrode of a lithium ion secondary battery, a lithium-containing composite oxide particularly LiCoO2 is well known. However, in recent years, for a lithium ion secondary battery for portable electronic instruments or for vehicles, downsizing and weight saving are required, and a further improvement in the discharge capacity of a lithium ion secondary battery per unit mass of the cathode active material (hereinafter sometimes referred to simply as discharge capacity) is required.
As a cathode active material which may further increase the discharge capacity of a lithium ion secondary battery, a cathode active material having high Li and Mn contents i.e. a so-called lithium rich cathode active material attracts attention.
As a lithium rich cathode active material, for example, the following have been proposed.
(i) A cathode active material having an α-NaFeO2 crystal structure, represented by Li1+αMe1−αO2 (wherein Me is a transition metal element containing Co, Ni and Mn, α>0, the molar ratio of Li to the transition metal element (Li/Me) is from 1.2 to 1.6, the molar ratio of Co to the transition metal element (Co/Me) is from 0.02 from 0.23, and the molar ratio of Mn to the transition metal element (Mn/Me) is from 0.62 to 0.72) (Patent Document 1).
(ii) A cathode active material represented by zLi2MnO3.(1−z)LiNiu+ΔMnu−ΔCowAyO2 (wherein A is at least one element selected from Mg, Sr, Ba, Cd, Zn, Al, Ga, B, Zr, Ti, Ca, Ce, Y, Nb, Cr, Fe and V, z is from 0.03 to 0.47, Δ is from −0.3 to 0.3, 2u+w+y=1, w is from 0 to 1, u is from 0 to 0.5, and y<0.1) (Patent Document 2).